Narcissus alkaloid, originally emerged as an antiviral agent from the screening program, has been found to possess anti-leukemic activity on Rauscher viral leukemia. The long-term treatment with the alkaloid showed the superior prolongation effect on the life span of advanced Rauscher leukemic mice having palpable splenomegaly in comparison with standard anti-leukemic drugs (cyclophosphamide, vincristine & 6-MP), interferon inducers (Poly I:C & Tilorone HCl) or some inhibitors of reverse transcriptase (Rifamycin SV, Ethidium Br. & Cinerubin-A). The response of the leukemic mice to produce antibody and interferon were not depressed by the long-term administration of the alkaloid. The combined treatment with cytoxan and 6-MP increases effectiveness. The alkaloid inhibits the growth of Rauscher virus in cell cultures, and the action mechanism seems to be the potent inhibition to reverse transcriptase. From the total alkaloidal fraction of narcissus bulbs, we have isolated 4 individual alkaloids (lycorine, pseudolycorine, tazettine and an unidentified Res.A-3).